1. Field of the Invention
This invention is directed to apparatus for correcting the indicated flow rate on meters used on fluid (principally gas) supply systems having measurement and/or delivery conditions varying substantially from meter calibration conditions.
2. Description of Prior Art
Insuring delivery of a desired flow rate of a fluid or fluid mixture is an important requirement of many fluid delivery systems. Generally, such systems will have an in-line meter to provide an indication of the flow rate of the fluid in appropriate units (volume per unit time, weight per unit time, etc.) and an operator will increase or decrease the rate of flow from the fluid source in order to achieve correspondence between the flow rate indicated on the fluid meter and a desired flow rate. Flow rate meters are calibrated to indicate flow rates for specific or "standard" conditions (eg. specific gravity, viscosity pressure, temperature, etc.) of the fluid medium. These fluid conditions are hereafter referred to as the "meter calibration" conditions. Variation of any of these fluid parameters under actual measurement conditions can introduce an error factor into the meter indication which may be of significance. Additionally, the conditions at the delivery point may be different from both the measurement conditions and the meter calibration conditions. The compressible nature of gases relative to liquids increases the magnitude of these errors relative to liquid supply systems, and it thus may be of crucial importance to correct these errors to as great an extent as possible.
Previous attempts to account for varying fluid conditions have usually concentrated on supplying correction devices to account for relatively small changes in the temperature and pressure of gases at the meter location. U.S. Pat. No. 1,585,859 to Huff shows a device for adjusting a meter scale to account for changes in barometric pressure and atmospheric temperature in a system used for feeding air to a compressor. While devices such as shown in Huff are adequate for applications wherein measurement conditions change only slightly from meter calibration conditions, they are inadequate to serve as correction devices in fluid supply systems wherein substantial changes in measurement and/or delivery conditions can occur.
One such application in which the fluid parameters may vary significantly from meter calibration conditions is related to supplying breathing gas from some source usually located above the water surface to a diver working underwater. In such underwater diving applications, the pressure at which the gas must be delivered increases dramatically with depth, with the pressure increasing by about 1 atmosphere for every 33 feet increase in water depth. In general, a diver uses about 2-5 cfm (cubic feet per minute) of gas at his ambient conditions. For a diver working at about 100 feet, and due to the compressibility of the gas, a mass of gas four times the amount at STP (standard temperature and pressure) conditions is required to be delivered to the diver containment in any given period of time. Since usual diving apparatus for supplying breathing gas from the surface includes a flow throttling device between the meter and the diver containment, it can be expected that gas conditions (pressure, etc.) in the diver containment could vary appreciably from the gas conditions existing at the point where the gas passes through the meter.
The present invention advantageously provides a correction device that can compensate for substantial parameteric changes at the point of measurement and/or at the point of delivery in a fluid supply system and still provide an adequate indication of the flow rate delivered. The correction device of the present invention can also handle a plurality of parameters influencing the indication on the fluid flow meter. And finally, the present invention provides an apparatus which may be purely mechanical for correcting flow rate meters, which apparatus can be fabricated economically and can function reliably in a highly moist and corrosive environment.
Additional advantages of the invention will be set forth in part and description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention may be realized and obtained by means and in combination particularly pointed out in the appended claims.